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The SIG MCX series features a short-stroke gas piston system to reduce recoil and improve the reliability of the weapon; this was based on the design of the earlier SIG MPX.[3] The MCX features a system that allows for conversion between 5.56×45mm NATO, .300 AAC Blackout and 7.62×39mm ammunition, using standard 5.56 mm STANAG magazines for 5.56×45mm NATO and .300 AAC Blackout, and specially designed STANAG-compatible magazines for 7.62×39mm.[3][4] The MCX is designed to deliver optimal performance with .300 AAC Blackout and an optional suppressor.[5]

The barrel's profile is tapered at the crown to allow the installation of muzzle devices and direct-thread sound suppressors without the use of washers that degrade performance and allows the devices to self-center on installation. The barrel can be changed in a matter of seconds to another length or a different caliber. Additionally the barrels are nitride coated for corrosion resistance.[6][7] It features hardened steel wear points.[3][6]

All MCX variants have a forend made of aluminum with a KeyMod system to add accessories. Controls are mostly ambidextrous including the charging handle but not the bolt release. Four types of stocks are available for the MCX carbine.[3][7]

SIG designed the upper receiver to be compatible with standard AR-15 and M16 lower receivers[3][6] with the help of an adapter.[8] The overall layout of the two rifles is similar.[9]

MCX SBR: Short-barreled rifle, with 9 inches (230 mm) barrel. Available with a safe/semi-automatic trigger group for U.S. civilian sales, or safe/semi-automatic/fully automatic trigger group for law enforcement and military sales. Under U.S. federal law, rifles with barrels shorter than 16 inches are Title II weapons, which are subject to federal restrictions, as well as being regulated by state laws.[3]

MCX Pistol w/ SBX: Semi-automatic pistol, with 9 inches (230 mm) barrel, SIG SBX stabilizing brace and safe/semi-automatic trigger group. This version fits the U.S. legal definition of a "handgun", in that it is only designed to be fired with a single point of contact with the shooter's body,[10] though in general terms, it is a compact carbine rifle, as it fires an intermediate round. The BATFE previously warned users that shouldering a weapon fitted with the SIG SBX, or a similar forearm brace, and not registered as a short-barreled rifle, constitutes the making of a short-barreled rifle, which is a Title II weapon.[11] However, as of April 2017, this is no longer the case.[12]

MCX Low Visibility Assault Weapon (LVAW): Short-barreled, suppressed, select-fire variant available only to law enforcement agencies and militaries. It is nicknamed "Black Mamba".[13][14]

^Federal Gun Control Act 1968 18 U.S. Code § 921 - Definitions, Cornell.edu, "(29) The term “handgun” means— (A) a firearm which has a short stock and is designed to be held and fired by the use of a single hand"